Ionic vacuum pumps



Jan. l0, 1967 D. GABOR Re. 26,138

IONIC VACUUM PUMPS Original Filed Nov. e3. 1960 IIA/en for PENN: 5 G0501? United States Patent O 9 Claims. (Cl. 313-7) Matter enclosed in heavy brackets l appears in the original patent but forms no part of' this reissue specification; matter printed in italics indicates the additions made by reissue.

This invention relates to ionic vacuum pumps in which the gas atoms or molecules are ionized by electronic impact, the ions being impelled by electric fields into contact with a getter material which removes them by absorption or adsorption.

Ionic vacuum pumps have been known for a long time in which electrons are made to oscillate between two electrodes at cathode potential which the electrons cannot reach, being prevented by a strong magnetic field from escaping sideways. Pumps of this type have been successfully embodied in sealed-ott vacuum devices. Their disadvantage is the large weight of the permanent magnets, and also their strong magnetic stray field which prevents their application in electron-optical devices sensitive to magnetic fields, such as cathode ray tubes or electronic cameras.

This invention relates to an ionic vacuum pump in which the movements of the ionising electrons are governed by purely electrostatic fields in such a way that, at least for the most part, they do not land on the appropriate electrode unless they have collided with a gas molecule.

According to the invention an ion-getter pump cornprises a boxor outer electrode presenting a surface of gettering material and having a rotationally symmetrical wall, said electrode being adapted to be maintained at a negative potential to serve as an ion collector, an axial electrode lying on the axis of symmetry of said wall and adapted to be maintained at a positive potential to serve as an anode, and means comprising an electron gun for producing ionising electrons, said gun being positioned to direct an electron stream tangentially into the device, the arrangement being such that in operation the ionising electrons are caused to travel in extended paths within the space encircled by said rotationally symmetric wall without the aid of a `magnetic field. ln the use of the device, the space encircled by said rotationally symmetric wall is, of course, in communication with the space to be evacuated, the outer electrode operating as the cathode and the axial electrode as the anode. By tangentially is meant in a direction tangential to any rotationally symmetric surface centered on the axis of symmetry of the outer electrode and removed from the axis, that is to say between the axial electrode and the wall.

The device is operated with a potential difference between the outer and the axial electrodes high enough for ionization and producing a generally radial electric field between the electrodes. The potential of the outer electrode is equal to or lower than the potential ofthe cathode in the electron gun, so that electrons fired from the gun cannot land on any point of the outer electrode. They are also prevented from landing immediately on the positive axial electrode by their momentum, which carries them past the axial electrode and makes them orbit around it, under the inuence of the electric eld, until they strike a gas atom or molecule, the electrons being ultimately collected at the axial electrode. Positive ions produced by the electron stream will move to the outer electrode which is made of or coated with gettering mate- Re. 26,138 Reissued Jan. 10, 1967 Mice rial which permanently absorbs or adsorbs at least a considerable proportion of the atoms formed by the neutralization of the ions.

The invention has particular advantages for use in association with electronic devices such as cathode ray tubes and the like, in which voltages suiciently high for the operation of the device are available, so that no separate supply is required. The invention is of special value in cathode ray devices such as projection tubes or colour tubes, in which gas development is strong owing to highpower operation, or to tubes containing complicated electrode structures or large `metal walls which cannot be perfectly outgassed. Moreover, the invention makes it ypossible to embody in such devices insulating materials such as silicones for example, which could not otherwise be employed in sealed-off high vacuum devices, because of their tendency to release gas continuously.

The invention will be better understood from the following description given with reference to the accompanying drawings in which:

FIG. 1 is a plan section of an ionic vacuum pumping device according to the invention.

FIG. 2 is an axial section thereof.

In FIG. 1 the outer or negative electrode is in the form of a box having a cylindrical wall 1, and end-plates 2 and 3, the latter being made of metal gauze to give access to the gas in the vacuum envelope (not shown) which surrounds the structure. The axial or positive electrode or anode is a wire or rod 4, supported on but insulated from the end-plates by insulating bushes 5. An electron gun 6 comprising a hot cathode 7, a grid 8 and an apertured anode 9 is mounted in the wall 1 with its axis directed in a tangential direction at radius R from the axis of the wall 1. The grid 8 is preferably of cathode f' potential and the anode 9 is at a potential high enough to draw a suiiicient electron current from the cathode, but preferably well below that of the anode 4. This choice of the potential arises from economy of power, and also from the requirement that the anode 9 shall not much disturb the rotational symmetry of the electric field inside the box.

A typical electron trajectory is indicated in FIG. 1 in dotted lines. It will be seen from this ligure that the electrons (due to their momentum and the generally radial electric field between the outer and the axial electrodes) orbit around the axis in approximately cycloidal trajectories, and that the radius R of the imaginary cylindrical surface to which the axis of electron gun 6 is tangent is several times larger than the radius of the wire or rod anode 4. Let V1 be the potential of the apertured anode 9; at the instant of leaving the aperture the electrons have an angular momentum around the axis proportional to RVl, which they will `preserve in their motion. Let r be the radius of the anode 4, and V2 its potential. If

no electron will be able to reach the wire 4, because the largest potential available in the device, which is V2, will not be sufficient to make the right hand side equal to the left hand side at the radius r, hence the radius r will not be accessible.

As an example, if V2: 10,000 volts, V1=100 volts, electrons will not be able to land on a wire with a radius r which is less than one-tenth of the initial radius R. In practice one must make an allowance for the disturbance of the rotational symmetry of the electrostatic field by the gun, and it is `preferable to make the wire radius smaller than that `given by the angular momentum condition. A lower limit is set by the heating of the wire 4, by the electron current to it due to electrons which have been in collision with gas molecules. This wire is therefore preferably made of tungsten or of titanium, or in the electrode wiiliin .mid Space which is nminmineri a! a pnviive pmenial din-ing use io Serve as an nnoiie wif/i mid negnlive and pm'irive poleniinls cmflrlisliing a ragiini electric field in Ille space /u'wcen .mid eleclrofiev, (nul means for direcrine ionizing eleeirons infn mid .vgnfcc will: a sufficient coniponcn of velocify mf'igen fo .snicl railinl eleerric )Ge/ci fo canse mid electrons to travel ibm/l Suid Second electrode in extended poi/is' williin .mici space rmtwiI/is'tanding ilmr said device does not comprise means opp/ying any strong magnerie eld acrosx .miil space.

8. An ionic vacuum pinnping device comprisingJ an outer electrode having c! .vin-joce of `een'cring nmlericil and embracing ci space znlapied io conm/'n cris molecules, soia' 0111er electrode being ninininincil at n ncgofive poientirll in use to .verve as an ion collector, a .second electrode wir/1in said Space which is rnainmineel ai a posilive poenlial in nie to Serre as an anode wiiii sniz! negative and posifive polentials esnlllilzing o rnrlial electric field in Ille space between .miel electrodes, and means for vroclncing ionizing electrons including nn electron gsm comprising an electron emiile." and means for (lirecn'ng electrons emitted ivy Snif. emitter into said eprice wif/i componenv of velocity tangent to said field such Ilmt Ille References Cited by the Examiner The following references, cited by the Examiner, are of record in the paemcd file 0f this paient 0r the original palcnl.

UNITED STATES PATENTS 2,523,541 11/1950 Pujas 3i3--62 X 2,689,928 9/1954 Youmaus 313--61 2,925,214 2/1960 Gurewitsch Q30- 69 JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Asfismnt Examiner. 

